As sensors for detecting micromaterials in liquid (e.g., proteins or pathogenic microbes in biosamples; and metal ions or organic molecules in water), sensors utilizing surface plasmon resonance (SPR) have been known (see NPLs 1 to 7). The above sensors utilizing surface plasmon resonance are commonly referred to as SPR (Surface Plasmon Resonance) sensors, and commercially available from many companies such as GE Healthcare, FUJIFILM Corporation, NTT Advanced Technology Corporation, and OPTOQUEST CO., LTD.
FIG. 1 illustrates an exemplary configuration of the most popular SPR sensor 200 in Kretschmann configuration. The SPR sensor 200 has a configuration including the thin metal layer 202 which is formed by vapor-depositing metals such as gold or silver on the glass substrate 201 and the optical prism 203 which is adhered to a surface of the glass substrate 201 opposite to a surface on which the thin metal layer 202 is formed; and has a function of polarizing laser light irradiated from the light source 204 by the polarizing plate 205 and irradiating the polarized light to the glass substrate 201 through the optical prism 203. The incident light 210A is made incident under a condition at which total reflection occurs. A surface plasmon resonance appears at a certain incident angle by an evanescent wave formed when the incident light 210A is transmitted to a metal surface-side. When the surface plasmon resonance appears, the evanescent wave is absorbed by surface plasmon, therefore, reflected light near the incident angle is significantly decreased in intensity. A condition under which the surface plasmon resonance appears varies depending on the dielectric constant in the proximity of the surface of the thin metal layer 202. Thus, when a sample to be detected binds to or adsorbs on the surface of the thin metal layer 202 to thereby change the dielectric constant, the reflection property of the incident light 210A also changes. Accordingly, the sample to be detected can be detected by monitoring, using the detector 206, a change in intensity of the reflected light 210B reflected from the thin metal layer 202.
The SPR sensor 200 detects the change in the dielectric constant in the proximity of the surface of the thin metal layer 202, so that it also can detect whether a certain substance accesses a metal surface (access), whether a substance which has been adhered to a metal surface desorbs from the metal surface, or whether a substance which has been present at a metal surface changes in property, in addition to adsorption of a sample to be detected.
However, in order to detect a property of a sample to be detected at a surface of the thin metal layer 202, it is necessary to move an optical system including the light source 204, alter an angle θ at which the incident light 210A is introduced to the thin metal layer 202, and then appropriately monitor the reflected light 210B by the detector 206, which causes complexity in configuration of the optical system and increases in size of a detection device.
A spectral measurement method has been reported in which an optical system in a SPR sensor is simplified and small-sized (see NPLs 6 and 7). FIG. 2 illustrates a schematic view of the SPR sensor 300 provided with the optical system which is employed in NPL 6. The incident light 310A is directed from the light source 301 to in front of the optical prism 303 via the optical fiber 302A, made into collimated light by the collimator lens 304, and then p-polarized by the polarizing plate 305, followed by being incident on the optical prism 303. This incident light 310A is irradiated to the thin metal layer 307 on the glass substrate 306, the glass substrate being arranged so as to adhere to the optical prism 303; and directed through the condensing lens 308 to the detector 309 via the optical fiber 302B, as the reflected light 310B which is reflected from the thin metal layer 307. Here, the photodetector 309 is provided with the spectroscope 309A, and has a function of measuring the reflection spectrum of the reflected light 310B. The SPR sensor 300 is similar to the SPR sensor 200 in that a change in the dielectric constant can be detected by measuring the reflection spectrum caused by the change in the dielectric constant in the proximity of a surface of the thin metal layer 307. However, it is difficult from the SPR sensor 200 in that the reflected light 310B is wavelength-resolved, and then measured for the spectrum thereof without changing the incident angle 310A to the thin metal layer by moving the optical system, which allows the optical system to be simplified and small-sized.
However, the SPR sensor utilizing the surface plasmon resonance has disadvantages in stability and sensitivity of measurements. Therefore, there is need to provide a highly stable and highly sensitive detection device.
An optical waveguide mode sensor has been reported which is similar to the SPR sensor in configuration and which also detects adsorption or change in the dielectric constant of a substance at a detecting surface of the sensor (see NPLs 1, 2, 8 to 19, and PTLs 1 to 5).
The optical waveguide mode sensor has been known to be capable of using an optical system equivalent to any optical systems used in the SPR sensors. FIG. 3 illustrates the optical waveguide mode sensor 400 having a similar configuration to the Kretschmann configuration. The optical waveguide mode sensor 400 uses the detection plate 401 consisting of the transparent substrate 401a (e.g., plate glass), the reflection layer 401b composed of a metal layer or a semiconductor layer coated on the transparent substrate, and the transparent optical waveguide layer 401c formed on the reflection layer 401b. Further, the optical prism 402 is adhered, via a refractive index-matching oil, to the surface of the detection plate 401 opposite to the surface on which the transparent optical waveguide layer 401c is formed. Light is irradiated from the light source 403, polarized by the polarizing plate 404, and then irradiated to the detection plate 401 through the optical prism 402. The incident light 410A is incident on the detection plate 401 under a condition at which total reflection occurs. Upon coupling of the incident light 410A with the optical waveguide mode (may be referred to as leaky mode) at a certain incident angle, the optical waveguide mode is excited to thereby significantly change the reflected light in intensity near the incident angle. Such a condition for exciting optical waveguide mode varies depending on the dielectric constant in the proximity of the surface of the transparent optical waveguide layer 401c. Therefore, the reflected light 410B changes in intensity when a substance is adsorbed onto, access, desorbs from, or changes in property on a surface of the transparent optical waveguide layer 401c. These phenomena such as adsorption, access, desorption, or change in property on the surface of the transparent optical waveguide layer 401c can be detected by measuring the change in intensity with the detector 405.
As disclosed in NPL 13 or PTL 5, a detection plate (SiO2/Si/SiO2 detection plate) used in the optical waveguide mode sensor has been proposed which includes silica glass (may be referred to as SiO2 glass, silica, or quartz glass) serving as a substrate, a silicon (Si) layer placed on the silica glass, and a silicon oxide (including thermal oxidized SiO2 or silica glass) layer placed on the silicon (Si) layer, and a highly sensitive and highly stable sensor can be achieved by using the detection plate.